Potato Greening in Supermarket Produce Displays
Light causes potatoes to turn green and accelerates the production of solanine, a poisonous and potentially fatal toxin also found in eggplants, tomatoes, and peppers.
Glycoalkaloid Enzymes in the Nightshade Family
When potatoes were first introduced to the Europeans, they were recognized as belonging to the nightshade family along with eggplants, tomatoes, peppers, and tobacco, and were considered poisonous. In fact, potatoes do contain poison in the form of glycoalkaloid enzymes which are always present within 3 mm (1/8 in.) of the surface of the potato, with the highest concentrations in the eyes or sprouts. Solanine is the most prevalent toxin in potatoes, and to a lesser degree solanine is also present in tomatoes, eggplants, and peppers, especially when the vegetables are green or not yet ripe. Other glycoalkaloid toxins found in the nightshade family include tomatine from tomatoes, and nicotine from tobacco. These natural poisons protect the plant from insects and other enemies.
Solanine Levels in Grocery Store Potatoes
In the past there was no regulation of solanine levels, which can vary depending upon the potato’s variety, age or maturity, and temperature, as well as the duration, intensity, and quality of light exposure. Commercially grown potatoes are now genetically controlled to have lower initial concentrations of solanine, but when potatoes are exposed to light, solanine levels can rise to ten times their original value.
Solanine levels in potatoes are associated with a green skin color caused by the formation of chlorophyll; the greener the potato, the more likely it is that the potato contains high concentrations of solanine and other glycoalkaloids. Chlorophyll is harmless, and is formed when the potato is exposed to sunlight or artificial light. Grocery store fluorescent lighting can induce potato greening in 12 hours to 5 days depending upon the type of potato, the light permeability of the packaging, and the ambient temperature, with potato greening occurring most quickly at room temperature (68°F) and in potatoes with thin skins.
Solanine does not need light to form, but once the potato is exposed to light and UV radiation, the rate of solanine formation increases. Moisture on the potato intensifies the effects of the light. Solanine production is a separate process from the formation of chlorophyll. However, if a potato is more than 5 percent green, the U.S. Department of Agriculture considers the potato damaged and less than US Grade #1.
Solanine Poisoning from Supermarket Potatoes
While death from potato poisoning is rare, eight ounces of a green potato can contain high enough levels of solanine to affect a 50 pound person, and 16 ounces could impact a 100 pound person. Symptoms of glycoalkaloid poisoning include gastrointestinal upset, headache, fever, convulsions, drowsiness, rapid breathing, delirium, and coma. Three to six milligrams of solanine per kilogram of body mass can be fatal.
Green potatoes often taste bitter, which is caused by the presence of solanine. However, toxic potatoes may not taste bitter, and bitter potatoes may not be toxic.
Boiling or steaming toxic potatoes prevents more solanine from forming but it only removes 30-40% of the toxin that has already formed. Solanine poisoning can be avoided by cutting off all green sections of the potato before cooking it. For people who tend to have allergic reactions, the entire potato should be discarded.
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